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September 5, 2002
A: FROM MENTOR MOLLY WILLIAMS. FOR BIO.
CLICK HERE.
When you close off the top of a soda straw that is partially
filled
with liquid, the liquid stays there because the air can't
get in.
Even though it looks like there's nothing in the top part
of the
straw, there's really air in there. If the liquid level fell
to the
bottom of the straw without allowing additional air into the
straw,
the air trapped in the top would have to expand (perhaps double
its
volume) and that expansion would cause the air pressure to
drop
inside the straw so that the air pressure outside would be
much
greater than inside. (Have you seen the ideal gas law that
says
pressure multiplied by volume divided by temperature is constant?
In this situation, if the gas volume increases then the pressure
has
to decrease). That difference in pressure between the inside
and
outside of the straw keeps the liquid column in place. You
can see
the same phenomenon when you pour liquid out of a narrow-necked
bottle. You can pour the liquid out smoothly as long as the
exiting
stream of liquid doesn't completely fill the neck, and air
can flow
into the bottle as the liquid leaves. If you hold the bottle
too
steeply, air can't flow in steadily, so it has to enter in
big
bubbles.
September 5, 2002
A: FROM MENTOR JOAN LUSK. FOR BIO. CLICK
HERE.
Congratulations - you've invented the pipette! in a thin tube,
the
pressure of the air at the open bottom is sufficient to support
the
weight of the column of liquid, so long as the top is closed
off. I
suppose the liquid sags down enough to create a partial vacuum
above
it, when the top is blocked, so the pressure above is less
than the
pressure of the external air below, enough less to support
the weight
of the liquid. You could see how much the level of the liquid
sags
by immersing a straw up to a mark, blocking the top, and withdrawing
the straw from the glass of liquid. If I'm right, the level
should
sag more for a larger weight of liquid - you could make a
thick sugar
or salt solution and test that, starting with the same length
of
liquid of different density, and the same length of air above
it.
Back in the old days, before we were so concerned with safely,
we'd
use calibrated glass tubes ("pipettes") just like
soda straws, suck
up the liguid into the tube and quick put a finger over the
open top
end. Then we could transfer a calibrated volume of the liguid
into
another vessel, wherever we needed it. Nowadays it is considered
too
dangerous to pipette anything by mouth - the pipette could
be
contaminated with stuff you don't want in your mouth, and
if you
sucked too hard you'd get a mouthful of the liquid you intended
to
pipette. But somehow we survived.... This site
http://www.woodrow.org/teachers/esi/2002/Biology/Projects/lab_skills/l
s7/
tells you a variety of different ways pipettes are now used
safely.
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